DIN-Rail Digital Input Module for Home Assistant — ALM-173-R1, 17 Opto-Isolated Inputs + 3 Relays

What You Can Connect

Alarm Logic

Tech Specs

Installation, Environmental & Mechanical

Home Assistant / Modbus / Web Config Integration

The ALM-173-R1 is designed as a Modbus RTU alarm endpoint that can be used with:

• HomeMaster MicroPLC and MiniPLC controllers (ESPHome based)
• Third‑party PLC and SCADA systems via Modbus RTU
• Home Assistant using an ESPHome‑based controller or Modbus integration
• Direct USB‑C configuration using a Chromium browser WebConfig tool

Internal firmware implements alarm groups (G1–G3 + Any), latching vs active‑while modes, relay follow/manual/override logic, optional local arming with zone types, and LED/button behaviour. Full Home/Away/Night alarm-panel modes and codes are intended for Home Assistant — not on-module. No EOL line-supervision (dry contact / SELV only). Configuration is stored in flash and automatically restored at power‑up.

Quick Setup Process (USB‑C WebConfig)

1. Mount & Power – Mount the module on a DIN rail, wire 24 V DC and RS‑485.
2. Connect USB‑C – Connect the module to a PC using a USB‑C cable.
3. Open WebConfig – Open the v0.2.0 ConfigToolPage in a Chromium-based browser (Chrome, Edge, Opera, Brave, Vivaldi; Chrome/Edge 89+, Opera 76+). Firefox: experimental only (Nightly + Web Serial). Safari/stable Firefox not supported.
4. Connect Device – Click Connect, select the serial device and wait for Active Modbus Configuration to appear.
5. Configure – Set Modbus address and baud, choose alarm modes for G1–G3.
6. Map I/O – Assign inputs to groups, configure relays, LEDs and buttons.
7. Save & Disconnect – Settings are stored in flash; disconnect USB‑C and hand over control to the Modbus master.

What WebConfig is and how it works: WebConfig is a browser-based configuration UI that runs in a Chromium-based browser using the Web Serial API (Chrome, Edge, Opera, Brave, Vivaldi; Chrome/Edge 89+, Opera 76+). Firefox Nightly with Web Serial is experimental; Safari and stable Firefox are not supported. It shows live Modbus address/baud in the header, streams a serial log, and lets you edit alarm modes, input groups, relay behavior, button actions and LED mapping. Changes apply immediately and are written to flash shortly after the last update so that configuration survives power cycles.

WebConfig Configuration Options

• Modbus address (1–247) and baud rate (9600–115200)
• Alarm modes per group: None / Active‑while / Latched‑until‑ack
• Digital inputs: Enable / Invert / Alarm Group / Zone type (Instant, Delayed, 24h-Tamper)
• Relays: Enable / Invert / Alarm Group (None/1/2/3), Power-on, Bell cut-off (s)
• Local arming (opt-in): entry/exit delay; Armed/Entry/Exit/Tamper status
• Buttons: Ack All, Ack G1–G3, or Relay 1–3 override
• LEDs: Steady/Blink, source Any, G1–G3, or relay override

uart:
  id: uart_modbus
  tx_pin: 17
  rx_pin: 16
  baud_rate: 19200
  parity: NONE
  stop_bits: 1

modbus:
  id: modbus_bus
  uart_id: uart_modbus
  turnaround_time: 100ms
  send_wait_time: 250ms

packages:
  alm1:
    url: https://github.com/isystemsautomation/homemaster-dev
    ref: main
    files:
      - path: ALM-173-R1/Firmware/v0.2.0/default_alm_173_r1_plc/default_alm_173_r1_plc.yaml
        vars:
          alm_prefix: "ALM#1"   # shown in Home Assistant
          alm_id: alm_1         # unique internal ID
          alm_address: 5        # Modbus address set in WebConfig
    refresh: 1d

The module needs a Modbus master on the bus. In a HomeMaster system that is the MiniPLC or the MicroPLC — both run ESPHome and connect to Home Assistant over the native API. To work out how many modules a panel needs and how wide it comes out, use the System Builder.

New to this kind of system? Two guides cover the ground: building a Home Assistant automation system and a wired ESPHome smart home.

Going deeper on this module: connecting dozens of wired sensors, what still works when Home Assistant is down and wiring a new house before the walls close.

FAQ

Power Supply

The ALM-173-R1 is powered from a regulated 24 V DC SELV supply connected to the V+ and 0V terminals. Internal converters generate the logic rails and isolated 12 V / 5 V sensor rails.

Power Input Specifications

Power path and protections (summary):

• 24 V DC input with reverse polarity and surge protection.
• Onboard conversion to 5 V and 3.3 V logic rails.
• Isolated 12 V and 5 V sensor rails (PS/1, PS/2) with PTC/fuse protection (for low‑power sensors only).
• SELV system; never use for mains or hazardous voltages.

For sizing the 24 V supply, consider base electronics, relay coil current and sensor rail consumption, with adequate headroom as described in the datasheet/README.

Inputs & Outputs

Digital Inputs (17 channels)

Digital input features:

• 17 independent opto‑isolated channels (IN1…IN17) referenced to isolated GND_ISO returns.
• Per‑input configuration: Enable, Invert (for NC sensors) and Alarm Group (None / G1 / G2 / G3) via WebConfig.
• Inputs feed the internal alarm engine, which generates Any Alarm and per‑group status bits on Modbus.
• Intended for SELV dry contacts or isolated low‑voltage signals only; never connect mains or non‑SELV circuits.

User Interface

Buttons and LEDs are fully configurable via WebConfig; the alarm engine combines inputs and groups to drive relays and indicators.

Connectors & terminal map

Top row (24Vdc | DIGITAL INPUTS | RELAY)

Bottom row (OUTPUT 12Vdc | OUTPUT 5Vdc | RS-485 | DIGITAL INPUTS | RELAY)

Ports & service interfaces

Housing notes

• RELAY1 and RELAY3 are on the TOP row, RELAY2 on the BOTTOM.
• Top relays read NC-C-NO; RELAY2 reads NO-C-NC. The contact order is reversed between rows.
• Inputs DI1-DI10 are on the top row, DI11-DI17 on the bottom.
• Sensor power: one isolated field domain (GND_ISO). +12 V on PS/1 and PS/2 is two fused branches of U58 (H2412S-2WR3) — 150 mA shared, not per terminal. +5 V (U59 H2405S-1WR3) is 200 mA and shares GND_ISO with the 12 V branches — not a second island.
• No end-of-line supervision. Wire alarm loops normally closed with invert so a cable break reads as an alarm.

Cabling and wirings

24 V DC Power Supply

Supply clean SELV 24 V DC to V+ and 0V, with an upstream fuse or breaker. Do not power field loads from the internal rails.

Digital Inputs (IN1…IN17)

Wire each input as a dry contact between INx and its matching GND I.x return. Do not inject external high voltage.

• 17 opto-isolated digital inputs (IN1…IN17) referenced to isolated GND_ISO returns.
• Intended for dry contacts / isolated low-voltage signals; do not connect mains or non-SELV circuits.
• Per-input configuration in WebConfig: Enable, Invert (for NC sensors) and Alarm Group (None / G1 / G2 / G3).
• Inputs feed the internal alarm engine, which generates Any Alarm and per-group (G1–G3) status bits exposed via Modbus.
• Use shielded twisted pair and keep DI wiring away from high‑dv/dt or high‑current cables for best noise immunity.

Relay Outputs

The ALM-173-R1 provides 3 SPDT mechanical relays for switching external loads using dry contacts. Each relay exposes NO / NC / COM terminals and is driven via isolated control circuitry from the MCU.

Rating: module output is 3 A @ 250 VAC (resistive, SPDT dry contacts). Relay components (HF115F class) are rated up to 16 A @ 250 VAC at chip level — that rating does not apply to the module. Use external contactors for higher power or inductive loads.

External protection required: Every relay output must be protected by an external fuse or circuit breaker sized according to the load and system limits. Relay output circuits are not internally fused, so external overcurrent protection is mandatory for safe operation.

Relay Specifications

Typical wiring using COM, NO and NC contacts. Relays are dry contacts and must be wired with external supply, fusing and snubbers where required.

• Each relay has COM / NO / NC terminals.
• Use NO for default-OFF loads, and NC for default-ON loads.
• Loads connected to relay contacts may carry hazardous voltage depending on your external wiring.
• Supports AC and DC switching; observe derating for DC and inductive loads as per contact manufacturer data.

Common protection warning: If a single common fuse or circuit breaker is used to protect multiple relay outputs, its rating must not be chosen by summing relay currents. The protective device rating must respect the per-output system limit and applicable standards.

For inductive or DC loads (contactors, solenoids, motors), use appropriate suppression (RC snubbers, MOVs, or flyback diodes). Loads exceeding recommended PCB current limits must be switched using external contactors or wiring that bypasses PCB copper paths.

Communication & Protocols

• Modbus RTU (RS‑485) – primary field bus for controller integration.

RS-485 / Modbus RTU

Terminal order differs across the HomeMaster range. Always read the silkscreen - do not wire by habit from another module. On this module the order is COM-B-A. Swapping A and B damages nothing but the node will not communicate. COM is required on every node.

All HomeMaster controllers and modules share the same RS-485 front end.

Bus wiring rules — apply to every device on the bus:

• One twisted pair for A/B, 120 Ω characteristic impedance.
• Run COM to every node. Required, not optional: the ports are not isolated, and COM is what bounds the common-mode voltage the transceivers see.
• Prefer one power supply for the whole bus, distributed in star topology. With separate supplies, additionally tie the 0 V references together at a single point.
• Bond the cable shield to cabinet PE at one end only. Never land a shield on A, B or COM.
• Where the bus crosses into a different electrical installation with its own earthing reference — a utility or billing meter, another building, another cabinet's PE system — fit an external galvanic RS-485 isolator at that boundary. The on-board components are transient protection, not isolation, and will not survive a sustained ground-potential difference.

Modbus Basics

• ALM-173-R1 operates as a Modbus RTU slave; the controller (PLC/ESPHome/SCADA) is the master.
• Address and baud are configured via WebConfig and stored in flash (restored at power‑up).
• Data model: Input Registers (FC04) @0..3 — bit-packed telemetry (inputs, alarms, LEDs, entry/exit/tamper, zone latch); Coils (FC01/05) @0..10 — stateful relay/armed outputs (0–3) + momentary ack/pulse commands (4–10). Configuration via WebConfig (USB-C), not holding registers. Identity @200: MODEL_ID=1, MAP_VERSION=4. Relay/Armed state is read via coil read-back (uniform with DIO).

Function Codes

• 0x04 Read Input Registers — state bitmasks @0..3; identity @200+.
• 0x05 / 0x0F Write Coils — stateful relay/armed (0–3), momentary ack and group pulse (4–10).

RS‑485 Wiring

Connect A, B and COM along the RS‑485 trunk. Use 120 Ω termination at both physical ends and avoid star topologies.

Termination & Biasing

• Terminate the RS‑485 line with approximately 120 Ω resistors at the two physical ends of the bus only.
• Do not terminate intermediate devices.
• Provide fail-safe biasing at the controller/master as recommended for Modbus RTU networks.

COM / Reference Ground

• Connect COM (reference ground) between all RS‑485 nodes when separate power supplies are used.
• This limits common‑mode voltage and helps prevent communication faults.

A common 0 V reference between controller and ALM module improves noise immunity and reduces RS‑485 communication errors.

USB‑C Interface

Cable Recommendations & Shield Grounding

General Routing Rules

• Route signal wiring (digital inputs, RS‑485, sensor rails) separately from relay load wiring and mains bundles.
• Avoid long parallel runs with contactor, motor or VFD cables; cross at 90° where needed.
• Use wiring ducts and strain relief to keep cables organised and protected.

Digital Inputs (IN1…IN17)

• Use 2‑core twisted pair per input (INx + GND I.x).
• Recommended size: 0.2–0.75 mm² (AWG 24–18).
• For long runs and noisy environments, use shielded twisted pair and route away from high‑dv/dt wiring.
• Inputs are for dry contacts / isolated low voltage only. Do not connect mains or high‑energy circuits.

RS‑485 (Modbus) Cable

• Use twisted pair for A/B; prefer shielded twisted pair.
• Implement a true daisy‑chain bus; avoid star topologies and long stubs.
• Terminate with 120 Ω at both physical ends of the trunk.
• Use one twisted pair for A/B and a third conductor for COM. COM must be run to every node.

Shield Grounding

• Terminate cable shields at one end only, typically at the controller or main panel earth bar.
• Bond shield to PE/EMC ground using a short, low‑impedance connection (clamp or grounding bar).
• Do not connect shields to signal terminals (A/B/COM, INx, GND I.x).
• Keep shield terminations short and separated from high‑energy conductors.

System Architecture & Pinout

System block diagram showing MCU, I/O expanders, opto‑isolated inputs, relays, sensor rails and RS‑485/USB interfaces.

MCU and connector pinout diagram with mapping of GPIOs to digital inputs, relays, LEDs and buttons.

Safety and Installation Notes

• Use only SELV 24 V DC power supplies. Never connect mains voltage to logic, inputs or relay terminals.
• Install inside a clean, dry, ventilated enclosure on a 35 mm DIN rail.
• Protect all wiring terminals against accidental contact using covers or ducts.
• Observe relay contact ratings; use external contactors and snubbers for inductive or high power loads.
• Separate field I/O wiring from mains and high‑energy conductors to reduce EMI and improve safety.
• Installation and servicing must be performed by qualified personnel familiar with panel building and low‑voltage safety.

Compliance & Certifications

The ALM-173-R1 module is CE marked. ISYSTEMS AUTOMATION S.R.L. (HomeMaster® brand) maintains the technical documentation and a signed EU Declaration of Conformity (DoC) available for download in the Documents and Resources section above.

• EMC Directive 2014/30/EU — EN 55032:2015 (Class B emissions), EN 55035:2017 (immunity), tested by Idvorsky Laboratories Ltd. (Job #1648)
• Low Voltage Directive 2014/35/EU — EN 62368-1:2020 + A11:2020, in-house dielectric and isolation testing by ISYSTEMS AUTOMATION compliance laboratory
• RoHS Directive 2011/65/EU — EN IEC 63000 technical documentation
• HomeMaster® — registered EU trademark (EUTM No. 019082911, EUIPO, registered 15 January 2025)

Licensing. Hardware design files — schematics, PCB layouts and bills of materials — are published under CERN-OHL-W v2. Firmware and the ESPHome integration packages are published under the MIT licence. Full terms are in the LICENSE files in the HOMEMASTER repository.